1. Field of the Invention
The present invention relates to a hollow graphene sheet structure and an electrode structure which can be widely used in electronic devices, etc., a process for the production thereof and a novel device comprising the electrode structure.
2. Description of the Related Art
A carbon nanotube comprises a graphene sheet (sheet-like structure of hexagonal network of carbon atoms) rounded in a hollow form. Since a carbon nanotube shows a high electrical conductivity despite its diameter as small as 1 to 50 nm in addition to its chemical stability, it has been under extensive study for its application to devices ranging from macroscale device such as discharge electrode to nanoscale electronic device. Since a carbon nanotube itself is tough besides being electrically conductive, it has been under study for application to support for reinforcing material or structure and hydrogen storing material utilizing the action of hollow structure.
Main applications for which such a carbon nanotube has been studied are single-wall carbon nanotube (SWNT) comprising one graphene sheet and multi-wall carbon nanotube (MWNT) comprising multiple layers of graphene sheet.
On the other hand, for the mass production of carbon nanotube, various methods such as arc discharge method and laser ablation method have been studied. However, since a carbon nanotube itself is a string-like material, its application is limited if it is used as it is. Therefore, a working technique such as cutting the carbon nanotube thus produced is required.
For example, in the case where a minute electronic device having a large amount of memory elements is prepared, even when the wiring itself can be formed by a carbon nanotube to have a reduced width, the size of the entire device can difficultly be reduced if the carbon nanotube itself is long.
As methods for working carbon nanotube there have been known a method which comprises cutting SWNT by oxidation with a strong acid (J. Liu et al., “Science”, 280, p. 1253 (1998)), a method which comprises cutting carbon nanotube in the form of a dispersion in a polymer by screw (M. Yudasaka et al., “Appl. Phys.”, A71, p. 449-451 (2000)), a method which comprises grinding and cutting carbon nanotube in the form of an ingot obtained by fixing in metal particles (Japanese Patent Laid-Open No. 2000-223004), and a method which comprises cutting carbon nanotube by ion irradiation (Japanese Patent Laid-Open No. 2001-180920).
However, the foregoing process involving the use of a strong acid is disadvantageous in that the entire carbon nanotube is damaged and the carbon nanotube must be heated over an extended period of time, making it difficult to obtain severed carbon nanotubes having desired properties and giving a drastically deteriorated productivity.
Further, this method cannot be used to cut a multi-wall carbon nanotube because many layers must be oxidized until the carbon nanotube is cut. In the method involving the dispersion in a polymer, complete cutting requires ultrasonic treatment, and a heating and combustion procedure is required to remove the polymer, adding to complicatedness.
In the method involving the use of metal particles, the carbon nanotubes thus cut cannot be generally used except in that carbon nanotubes protruding from the surface of the ingot are used as emitter.
In the method involving ion irradiation, it must use an acceleration voltage of from several kilovolts to scores of kilovolts requiring a large-scale device and two steps, i.e., step of forming an unbonded site on carbon nanotube by ion irradiation and step of cutting carbon nanotube by reaction of the unbonded site with oxygen or hydrogen, adding to complicatedness.
As mentioned above, these known working methods have disadvantages.
On the other hand, a method which comprises etching the surface of graphite with a particulate gold as an oxidizing catalyst is described in H. Watanabe, “Jpn. J. Appl. Phys.”, 32, p. 2809 (1993).
Since a carbon nanotube is subject to contamination by a large amount of impurities such as amorphous carbon and graphite during its production process, E. Mizoguchi et al. proposed as a method for extracting carbon nanotube by referring to these literatures a method which comprises mixing gold particles with a surfactant, heating the mixture in an oxygen atmosphere so that the amorphous carbon and graphite are oxidized and decomposed away to leave as an extract SWNT which is stable as compared with these impurities (E. Mizoguchi et al., “Chem. Phys. Lett.”, 321, p. 297-301 (2000)).
It is described that in this method, heating at high temperature causes SWNT itself to be oxidized and decomposed away. Accordingly, the combination of a particulate gold and a carbon nanotube allows the removal of impurities but cannot be expected to be applied to shaping of carbon nanotube. There are neither suggestions on this application.
In order to prepare a device comprising carbon nanotube or provide a carbon nanotube itself with desired functions, it is desirable to simply work the carbon nanotube itself without impairing the properties thereof. The methods which are known at present have the foregoing advantages and disadvantages.
In order to use a carbon nanotube for various purposes, a carbon nanotube having a new shape is required.
On the other hand, in order to use a carbon nanotube as an electrode for connecting elements, it is a great task how the carbon nanotube itself is disposed. In other words, the carbon nanotube is very fine and thus can be difficultly handled. For example, when carbon nanotubes are used as at least two electrodes, it is extremely difficult to dispose the two carbon nanotubes such that a minute gap occurs according to the minute node space of electronic elements having a reduced size.
The invention has been worked out in the light of the foregoing problems. An aim of the invention is to provide a novel hollow graphene sheet structure and electrode structure by cutting or otherwise working a hollow graphene sheet material comprising carbon nanotube with a lessened damage on the structure of the hollow graphene sheet material. Another aim of the invention is to realize the application of the electrode structure to a minute device of hollow graphene sheet material such as carbon nanotube.